MHC class I genes encode transplantation antigens that play important roles in immune responses. Transcription of major histocompatibility complex (MHC) class I genes is regulated by the conserved MHC class I regulatory element (CRE). In the MHC CRE there are at least three sequences, regions I, II and III that bind nuclear factors. Region I binding activity is expressed only in tissues that express MHC class I genes at high levels, while the region II factor is detected in various tissues irrespective of the MHC class I gene expression. While proteins that bind to various regions of the MHC CRE are known, a protein that binds selectively only to the native region II sequence, but not to the other sequences of the MHC CRE, has not heretofore been known or described. This family of receptors is the subject of the present invention.
More particularly, these nuclear hormone receptors belong to the steroid/thyroid hormone receptor superfamily of transcriptional regulators. The compositions of this invention are retinoid X receptors (RXRs). They are highly conserved proteins and the sequences provided herein are useful for identification and isolation of homologous genes in all mammals.